How long do structures take to form in transpression zones? A cautionary tale from California

It is generally assumed that individual sets of coplanar and colinear deformation structures form together during events that are of relatively short duration (1–5 m.y.). The record of deformation in a sequence of Late Cretaceous to Holocene sedimentary rocks from the northern Salinian block of California spans at least 30 m.y. and illustrates that this assumption is some- times incorrect. At different localities, geometrically and kinematically identical contractional structures either predate or postdate local unconformities of varying age within the succession, so that it is possible to define at least four chronologically distinct, but otherwise indistinguish- able, deformation episodes. In the absence of the unconformities, the punctuated nature of the deformation would not be apparent, therefore suggesting that subparallel structures may form during successive, distinct deformations spread out over long time periods. In the northern Salin- ian block, the inferred contractional strain field is approximately normal to the adjacent San Andreas fault and appears to have been consistently oriented in this direction during deforma- tion events recorded over the past 30–45 m.y. The strain pattern is most easily explained by effi- cient partitioning of transpressional strains into fault-normal shortening and right-lateral fault- ing during episodic regional deformations. We propose that reactivation of preexisting structural anisotropies controls the observed partitioning of deformation in many transpression zones.